This invention relates to an infrared radiation detection device for detecting radiation in a given wavelength range of the kind comprising a flexible thin film of polymer material having pyroelectric properties on opposite sides of which electrodes are disposed.
Pyroelectric infrared radiation detector devices are used in a variety of applications. Their characteristics make them particularly attractive for application in remote switching systems, intruder detection systems and in movement sensing generally. In such applications, the detector device responds to a moving object by detecting the flux change produced in passing through the device's field of view. In the case of the object being a person, the infrared radiation emitted by the moving person or body part is converted by the detection device into an electronic signal which can be used, for example, to actuate an alarm or to switch lights on and off.
In a known form of detector device suitable for intruder detection purposes, a pyroelectric polymer dielectric film is sandwiched between a pair of electrodes which collect electrical charges developed at the surfaces of the polymer film in response to a temperature change in the film resulting from incident infrared radiation. The device includes an FET for impedance matching purposes whose gate is coupled to one electrode and from whose source the detector output signal is obtained. The polymer film typically has a thickness of between 12 and 20 .mu.m to provide small heat capacity, and quick responsiveness.
For use in an intruder detection system, it is desirable that the detector device be capable of sensing infrared detection in the approximate wavelength range 5 to 15 .mu.m.
One problem associated with infrared radiation detector devices is ensuring sufficient absorption of incident radiation at these wavelengths. Pyroelectric materials suitable for detector devices often have relatively low absorption over at least part of the wavelength range with which they are intended to operate. Also, the thinner the pyroelectric material, the lower will be the overall absorption of the incident radiation by the material. Therefore, it has been customary to improve absorption characteristics of the detector device by techniques such as the provision of black absorber coatings, providing a substantially wavelength independent response. Such blackening may take the form of blackening of one of the device's electrodes. In order to enhance the sensitivity of the detector device for a particular radiation wavelength or wavelength range, a coating of selective radiation absorbing material may be applied providing the required filtering characteristic. Radiation of the wavelength range of interest is then absorbed in this coating and the heat produced conducted to the pyroelectric material. These spectrally selective absorption coatings are applied to the outer surface of one of the detector devices's electrodes.
The provision of such absorptive coatings complicates manufacture of the devices and, particularly with selective absorbers, are comparatively costly to produce.
It is an object of the present invention to provide an infrared radiation detection device which has selective wavelength response characteristics and which is simpler, and less costly, to produce.